Tue. Jul 5th, 2022

In a stinky, swampy mangrove forest in the French Caribbean, a strange giant lurks among the fallen leaves.

It looks more like a vermicelli noodle than something out of The Creature from the Black Lagoon and it has an insatiable appetite for sulfur – the stuff that gives rotten egg gas its unmistakable stench.

Meet Thiomargarita magnifica, the largest bacterium that scientists have found so far.

At a whopping 1 to 2 centimeters long, this giant microbe is bigger than a fruit fly and can be seen with the naked eye, according to research published today in Science.

“It’s about 5,000 times bigger than most bacteria,” said lead author Jean-Marie Volland, a microbiologist at the Lawrence Berkeley National Laboratory in California.

A giant hiding in plain sight

Tree roots underwater
This mangrove forest is home to the largest known bacteria. (Supplied: Pierre Yves Pascal)

Discovering the largest-known bacterium was the last thing on Olivier Gros’ mind when he was hunting for microbes in the mangrove forests of Guadeloupe in the French Caribbean almost two decades ago.

While collecting samples from the murky water, Professor Gros noticed a long white filament clinging onto a leaf.

The marine biologist decided a closer look was in order, and took the hair-like filament back to his lab at the University of the Antilles in Guadeloupe and viewed it under a microscope.

“It was so huge,” Professor Gros said.

Professor Gros could not see any of the typical cellular features you would find in eukaryotes, the broad group that includes plants, animals and fungi.

For a start, there were no mitochondria – the cellular machines that produce energy.

There were also no nuclei, the tiny structures within our cells that hold genetic information.

Instead, the thread-like organism looked more like a single cell than a chain of hundreds.

“It was just something strange,” he said.

Long gray filament under microscope
The bacterium is thousands of times bigger than most other bacteria.(Supplied: Olivier Gros)

But an initial genetic analyzes revealed that the oddball organism’s makeup matched with Thiomargarita, a genus that includes bacteria that feed on sulfur.

So Professor Gros and his team tentatively named the microbe Thiomargarita magnificaa nod to “magnus”, the Latin word for “great”.

Bizarre inside and out

When Dr Volland joined the lab a few years later, his heart was set on studying the “macro-microbe” and confirming that it was indeed a bacterium made up of one cell.

The team collected more samples from the mangrove forests in Guadeloupe and used a range of powerful microscopy techniques to look at the filaments in three dimensions.

When Dr Volland and his team zoomed in on these single-celled threads and scanned their entire length, he could not see segments you would expect to see in a multicellular organism.

He also saw tiny seed-like compartments containing the bacterium’s genetic blueprint.

These “pepins” were a bizarre feature, because bacterial DNA typically floats freely inside cells instead of being neatly bundled inside cellular containers as it is in humans, plants and animals.

“This has never been observed in bacteria before,” Dr Volland said.

“It’s actually something that is characteristic of complex organisms.”

Diagram of bacteria cell
In most bacteria, DNA floats freely inside the cell.(Getty Images: Vitalii Dumma)

These packages of genetic material could have allowed the bacterium to grow to its massive size, said Ashley Franks, an environmental microbiologist at La Trobe University who was not involved in the study.

“We used to think that [bacteria] were always like a soccer ball and they were set to a certain size, “Professor Franks said.

“Everything just mixed together in the middle and that’s how you got all the functions.”

This was thought to be why most bacteria have stayed small, as growing too big would throw their inner chemistry out of whack, Professor Franks said.

But because T. magnifica‘s insides are relatively organized and somewhat separated, it was able to grow to its huge size without losing its chemical balance.

“It starts to tell us about how you go from something basic like a bacterium to something much more complex,” Professor Franks said.

A library of genetic possibilities

Next, the team sequenced T. magnifica‘s genome using five of the filaments they collected.

The genome itself was huge and contained roughly 12,000 genes – that’s three to four times more than most bacteria, Dr Volland said.

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